1. Field of the Invention
The invention relates to a vibration driven motor and, more particularly, to a driving circuit for a vibration driven motor.
2. Related Background Art
In recent years, a vibration driven motor called an ultrasonic motor or a piezo-electric motor has been developed and has been put into practical use by the same applicant as the present invention or the like. As already well-known, the vibration driven motor is a new type motor of the non-electromagnetic driving type which is constructed in a manner such that by applying an alternating voltage to an electro-mechanical energy conversion element such as piezo-electric element, electro-strictive element, or the like, a high frequency vibration is caused in the above element, and a vibration energy is taken out as a continuous mechanical motion. The vibration driven motor is mainly classified as a standing wave type motor or a travelling vibration type motor according to the kind of vibration which occurs in the piezo-electric element or electro-strictive element.
FIG. 5 schematically shows a driving circuit of a conventional vibration driven motor of the travelling vibration type. Reference numeral 1 denotes a vibrator comprising a coupling member of a piezo-electric element or an electro-strictive element and an elastic member; 1a, 1b, 1a', and 1b' indicate driving electrodes for applying an alternating voltage to the piezo-electric element or electro-strictive element; 2 indicates an oscillator to generate the alternating voltage; 3 indicates a phase shifter of 90.degree.; and 4a and 4b indicates amplifiers.
As shown in the diagram, the above motor is a two-phase driven motor and there is no difference between the first and second phases except that the phase of alternating voltage which is applied to each phase is shifted by .+-.90.degree. by the 90.degree.-phase shifter 3. Therefore, only the one-phase portion shown by reference numeral 5 will now be described.
FIG. 6 shows an equivalent circuit of the first phase portion 5. Reference numeral 6 denotes an amplifying section; and 7 denotes an equivalent circuit of the vibration driven motor. The equivalent circuit 7 comprises: an RLC serial circuit (which is constructed by an equivalent resistor 7a of a resistance value R.sub.m, an equivalent coil 7b of a self-inductance L.sub.m, and an equivalent capacitor 7c of an electrostatic capacitance C.sub.m) of the mechanical vibrating portion; and a capacitor 7d of a specific electrostatic capacitance C.sub.d of the vibrator 1 which is connected in parallel with the RLC serial circuit.
However, hitherto, assuming that the alternating voltage which is applied is set to V.sub.i and the voltage across the driving electrodes 1a and 1a' is set to V.sub.O, it is generally necessary to apply a voltage within a range from tens of volts to hundreds of volts to the vibration driven motor. Therefore, a voltage step-up ratio n=.vertline.V.sub.0 /V.sub.i .vertline. exerts a large influence on the characteristics of the motor.
The vibration driven motor increases and controls an amplitude of vibration wave by using the mechanical resonance. For this purpose, an alternating voltage of a frequency near a mechanical resonant frequency ##EQU1## is applied. However, the equivalent circuit constants R.sub.m, L.sub.m, and C.sub.m of the mechanical vibration of the vibration driven motor vary depending on an environmental temperature of the motor, a magnitude of amplitude, or the like. Therefore, in the frequency characteristics of the equivalent circuit shown in FIG. 6, an input electric power and an output power of the motor vary widely. A driving circuit suitable for the above frequency characteristics is needed.